Howard Hinnant <howard.hinnant <at> gmail.com> writes:
[thread.req.timing]/2 says:
The member functions whose names end in _for take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions. [ Note: Implementations are not required to use a monotonic clock because such a clock may not be available. — end note ]
In English: wait_for should always use a monotonic clock under the hood, unless the platform doesn't have one.
Ah! Enabling and enforcing the use of the monotonic clock for relative timeouts feels instinctively correct; and just what I was after.
Templating the condition_variable on clock_type was specifically avoided for std::condition_variable because a condition_variable often has more than one waiting client at a time, and it was felt that those different waiting clients should be able to use different clocks simultaneously when waiting on a (absolute) time_point.
Of course; you are correct. I was being misled by the existing pthread implementation of boost::condition_variables and boost::chrono which currently enforces a single clock on all clients of a condvar. This limitation seems to have been inherited from pthread_cond_timedwait and pthread_cond_init which do not provide the fexibility for mixing of clock types for waiting clients of a cv. (The pthread_cond clock type is fixed at condvar creation). Fixing this limitation would now seem to be a problem for the pthread platform implementation, rather than the boost API and the standard. I'll propose some changes to boost::chrono pthread implementation in line with the draft standard. Many thanks for the very useful information. regards Morgan